Re: [PATCH] -Walloca-larger-than with constant sizes at -O0 (PR 100425)

[Martin Sebor <msebor@gmail.com>]

On 5/5/21 1:32 AM, Richard Biener wrote:
> On Wed, May 5, 2021 at 4:20 AM Martin Sebor via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> Even when explicitly enabled, -Walloca-larger-than doesn't run
>> unless optimization is enabled as well.  This prevents diagnosing
>> alloca calls with constant arguments in excess of the limit that
>> could otherwise be flagged even at -O0, making the warning less
>> consistent and less useful than is possible.
>>
>> The attached patch enables -Walloca-larger-than for calls with
>> constant arguments in excess of the limit even at -O0 (variable
>> arguments are only handled with optimization, when VRP runs).
> 
> Hmm, but then the pass runs even without -Walloca or -Walloca-larger-than
> or -Wvla[-larger-than].  It performs an IL walk we should avoid in those
> cases.
> 
> So the patch is OK but can you please come up with a gate that disables
> the pass when all of the warnings it handles won't fire anyway?

-W{alloca,vla}-larger-than=PTRDIFF_MAX are enabled by default so
the pass needs to do walk.

FWIW, it would make sense to me to consolidate all the checking of
calls for arguments with excessive sizes/values into the same pass
and single walk (with code still in separate source files).  As it
is, some are done in their own passes (like alloca and sprintf),
and others during expansion (-Wstringop-overflow), and others in
calls.c (-Walloc-size-larger-than).  That leads to repetitive code
and inconsistent approaches and inconsistent false positives and
negatives (because some are done at -O0 but others require
optimization).

Martin

> 
> Thanks,
> Richard.
> 
>> Martin